The invention relates to a device and a method for making interferometric measurements.
Interferometric and polarimetric sensors have output signals in which there is a sin.sup.2 and cos.sup.2 -dependent variation of the phase difference between the measured light wave and the reference light wave, induced by the measured quantity. The signals are therefore non-linear and periodic, which makes evaluation difficult.
For this reason, special opto-electronic methods are used to obtain a linear, unambiguous relation between the measured quantity on the one hand and the interference signal on the other hand.
Fiber-optic interferometric sensors have to be read out by special demodulating methods for stabilising the working point when reading out small signals and for obtaining precision and the sign of the change in the measured value in the case of large phase shifts. This is necessary because the periodic interference signal is subject to facing as a result of temperature drift in the case of small signals. At maximum and minimum intensity the measured signal vanishes. Another effect of periodicity at large phase shifts is an ambiguity of the output signal with respect to the sign. A photodiode connected to the interferometer output for further electronic processing records the same light/dark change irrespectively of the direction of the change in the measured quantity. Numerous attempts at solving this problem have already been made, e.g. by simultaneous use of two light sources with different wavelengths, suggested e.g. by Peter de Groot and Stanley Kishner, "Synthetic wavelength stabilisation for two-color laser-diode interferometry", in APPLIED OPTIC 30 (1991) 4026-4033. When, in the cited proposed system, the signal determined at one wavelength .lambda..sub.1 has drifted to the minimum sensitivity, the signal determined at the second wavelength .lambda..sub.2 is exactly at the point of maximum sensitivity for small signals. A suitable combination of the two outputs or a periodic change-over between the two wavelengths can be used e.g. for the purpose of carrier-frequency modulated phase measurement and for a read-out free from signal fading and/or with the correct sign. The difference in wavelengths has to be adjusted in each case at a special sensor so that the phase difference between the two intereference signals is exactly 90.degree. (.pi./2). This results in so-called quadrature signals: EQU (1-cos.PHI.(.lambda..sub.1)), (1-sin.PHI.(.lambda..sub.2)).
The disadvantage of this idea is the relatively high cost, due to use of two light sources and the associated operations.
For completely different purposes in microscopes, Katherine Creath, "Step height measurement using two-wavelength phase-shifting interferometry", in APPLIED OPTICS 26 (1987), 2810-2816 proposes changing the filters used manually in the case of wide-band white light for the purpose of interferometry at two wavelengths. This is impracticable and does not result in usable evaluating systems, and there are higher-order effects.